Thursday, August 6, 2015

The most energetic neutrino - in picture form

This is an event view of the highest energy IceCube neutrino that I mentioned in my last post, as presented at the 2015 International Cosmic Ray Conference.  Each sphere is one optical sensor; the colored spheres show modules that observed light from this event.  The sizes of the spheres show how many photons each module observed, while the color give some idea of the arrival time of the first photon, from red (earliest) to blue (latest).

It is easy to see that the neutrino is going slightly upward (by about 11.5 degrees), so the muon cannot be from a cosmic-ray air shower; it must be from a neutrino.  The fact that the muon comes from just below the horizon is not surprising.  At PeV energies, neutrinos interact more than at lower energies, so the Earth is opaque.  So, we do not expect to see near-vertical upward-going neutrinos with PeV energies.

The measured visible energy is 2.6 +/- 0.15 PeV (1 PeV = 10^{15} eV).  This is the energy actually seen in the detector.  It does not include the energy lost by the muon before it reaches the detector,  the energy carried off by the outgoing muon, or the fraction of the neutrino energy that was transferred into a hadronic shower, rather than the observed muon.  So, the actual neutrino energy is a multiple of this. 



Wednesday, July 29, 2015

The most energetic neutrino yet!!

IceCube has just announced the observation of our most energetic neutrino yet.  The event was in the form of a through-going muon, which means that we saw a piece of the track in our detector, but the track both originated and ended outside of the enclosed volume.  So, we cannot measure the total energy of the neutrino.  Instead, we measure the specific energy loss (energy loss/distance, or dE/dx).  From that, we can estimate the muon energy, in the detector, and, from that, we can, based on an assumption of the neutrino energy spectrum, estimate the probability that neutrino had a range of energies.   We are still working on estimating the neutrino energy, but the total energy loss visible in the detector was 2.6 +/- 0.3 PeV.  This is, of course, a lower limit to the neutrino energy, making it clearly the most energetic neutrino yet observed.  Typically, one expects the neutrino energy to be a couple of times higher than the muon energy.

The event came from the Northern Sky (coordinates R.A.: 110.34 deg and Decl.: 11.48 deg), and we currently estimate the average angular resolution to be 0.27 degrees.   Because it was upward-going, we know that it must be a neutrino, and not cosmic-ray muon background.

The event was recorded on June 11, 2014 (see the link for details), just over a year ago.  We are clearly getting better at processing and analyzing our data more quickly, but there is still room for improvement.

The event was announced as an "Astronomers Telegram," (ATel) a brief announcement which can be issued quickly.  The main purpose of ATels is to get word out quickly about a new transient phenomena (gamma-ray burst, nearby supernova etc.), so that other astronomers can point their telescopes in right direction while the phenomena is still going on.  In our case, there is no reason to expect this to be (or not be) from a transient phenomena, and, if it was, it is probably over now.  But, we are releasing the coordinates now, so that other observatories can see if they can find anything unusual in that direction.   The event is relatively near the equator, so it should be visible from most large observatories.

More information later (probably after the Intl. Cosmic Ray Conference, July 30-Aug. 6th).

Note Added (July 30th).  Unlike Bert, Ernie and Big Bird, we have named this event after a Muppet. 

Wednesday, July 15, 2015

Getting Science Right

Every once in a while, a scientific scandal makes big news.  Someone faked doing an experiment, or grossly misinterpreted their results, or failed to reproduce someone elses important result.  Particularly in medicine, this can have big consequences.

Unfortunately, these scandals are often blown out of proportion, with insinuations that many scientists are dishonest.   At least partly because of this, scientists are paying increasing attention to irreproducible results.  There is a blog, "Retraction Watch" which is devoted entirely to scientific papers that have been formally retracted.  Some common problems are plagiarism (including self-plagiarism) or apparently faked results (particularly manipulated images).  Honest scientific mistakes (i.e. missed minus signs, etc.) also make an appearance, as does occasional subversion of the peer review process.    These problems are real, but it is important to keep them in perspective.  Retraction watch typically posts 1-3 retractions/day, out of hundreds of thousands of scientific papers published each year.  This is a very miniscule percentage.  Although Retraction Watch probably doesn't catch every retraction, they do appear to be very efficient at finding them.

Many of these errors are caught rather quickly, by other scientists.  Most important retractions occur within a year or two.   Pubmed, an online library of medical literature, run by the National Institute of Health recently (2013) opened Pubmed commons, where readers can comment on the scientific literature; suspect images and other visible problems can be (and are) vigorously discussed.  

A bigger problem may be papers that are just not reproducible, for reasons that are not clear.  This is mostly an issue for biology and medicine, fields that deal with complex systems (large molecules, cells, humans), where .   At least according to some reports, like this article in the New Scientist, this is an epidemic problem, affecting a large fraction of published papers.  This track record is a good reason to take the latest medical advice with at least a small grain of salt.  However, even here, the scientific record is generally self-correcting, albeit most slowly.  Science builds on previous results, and you can't build much on a cracked foundation. Darwinian evolution gradually weeds out bad conclusions.

As a more quantitative science, physics suffers less from irreproducibility than biology.  It is far easier to quantify the uncertainties in a neutrino energy measurement than in, for example, unknown contaminants in a reagent used in a biology experiment.    Over the past decades, physics has also taken increasing efforts to eliminate sources of unconscious bias.  In many experiments (IceCube included), most analyses are done in a 'blind' manner, whereby the analyst prepares his analysis using simulated data, and a small fraction of the real data.  Only after the analysis procedure is fixed, and reviewed by the collaboration, is the real data analyses.  This avoids any tendency to zero in on fluctuations in the data (the 'look here' phenomena).  So, when choosing a list of possible neutrino sources to analyze, we won't unconsciously pick one(s) that correspond to upfluctuations in the data.   As a result of these practices, particle and nuclear physics have a pretty good (but not perfect) record with being able to reproduce previous results.













Wednesday, March 4, 2015

Bert and Ernie's less energetic cousins

Since the original observation of Bert and Ernie, followed by Big Bird, we have been trying to learn all we can about our extra-terrestrial neutrino signal.   As previously mentioned, we have seen no statistically significant sign of any clustering indicative of a single source or multiple sources.  So, our efforts have been focusing on ways to better characterize the signal.   Today, I want to tell you about two efforts.

One obvious way to progress is to study the signal at lower energies (at higher energies, we don't see anything).   This is, however, easier said than done, since the backgrounds rise steeply at lower energies.   One way to handle that is to use larger and larger veto regions as the energy drops;  this leaves a smaller and smaller fiducial (active) detector volume, but the signal should also rise at lower energies.  Jakob Van Santen pursued this route, in an IceCube paper that was published in Phys. Rev. D. in January, and available on the Cornell preprint server, as arXiv:1410.1749.    

The plots below show the results of that study, for the Northern and Southern skies respectively.   The backgrounds get larger at lower energies.  In the Southern sky, there are two types of backgrounds: penetrating muons and an irreducible background of atmospheric neutrinos, while, in the North, only the neutrinos are present.  However, in both cases, it is possible to measure the astrophysical component down to deposited energies of a few TeV.   Here, "deposited energy" means the energy visible in the detector. 
The measured flux is consistent in both hemspheres, and is well fit by a power-law spectrum:                      phi ~ (E_nu)^-p, where p, the power law index, is 2.46 +/-0.12.  The spectrum is significantly softer (i.e. has more low-energy events and fewer high-energy events) than the standard benchmark spectrum, which is dN_nu/dE_nu ~ (E_nu)^-2.  This is not a surprise;  most of us thought that the -2 index was based on a simplified model which would not survive an encounter with data.

The second analysis, by Gary Binder,  also looked at the energy spectrum (with similar results), and also looked at the neutrino flavor ratio: how many of the neutrinos are electron neutrinos (nu_e), vs. muon neutrinos (nu_mu)  vs. tau neutrinos (nu_tau) . It is available on the Cornell preprint server, as arXiv:1502.03376.   It found a similar spectral index (as have other IceCube studies). 

The flavor ratio is somewhat tricky, in that there are three different types of neutrinos which interact via two topologies: long muon tracks from nu_mu, and roughly spherical showers, from nu_e and all flavor charged-current interactions.  In this energy range, 83% of nu_tau produce showers, while 17% of them include muons.  So, there is some ambiguity.  Gary presented his results as a triangle:
Each point in the triangle corresponds to a specific nu_e:nu_mu:nu_tau ratio; the fraction can be found by reading across to the right for the nu_tau fraction, downward to the right for the nu_tau fraction, and upward to the left for the nu_e fraction.  The four symbols in the legend correspond to four different models for neutrino production in a source: via pion decay to muons, via pion decay to muons which lose energy rapidly, via neutron decay, and via pion decay to muons which gain energy before they decay.  At the source, these models predict quite different flavor ratios.  However, the sources are very distant, and the neutrinos will oscillate during their trip, arriving at something much closer to an equal mix of flavors. 

The current analysis find a best fit indicated by the cross near the lower left.  However, the confidence levels (shown via colors) show that all four models can adequately fit the data.  However, the analysis does rule out non-standard models (not shown here, but discussed in the paper) such as some involving sterile (non-interacting) neutrinos.



Tuesday, February 3, 2015

The 2014/2015 ARIANNA field season - part 2

This is part 2 of the post by Joulien Tatar about the 2014-2015 (really, 2014) field season, with more photos by Chris Persichilli.





Including a few stormy days that kept us in our tents, we installed four new stations in about a week.  There were three stations already installed from previous years so technically we were done with the hexagonal station array construction.  However, given that we were ahead of schedule, we decided to unbury two of the three pre-existing stations in order to upgrade their hardware and conduct some performance studies.  One of the stations was left completely unhampered with it for a third consecutive season to continue studying long term system performance.  Then disaster struck - we ran out of good coffee!  For an avid coffee drinker, like myself, this was a nightmare.  The panic subsided after a desperate scramble turned up a can of instant coffee stashed away in one of the food boxes...phew...

Most of our remaining time on site was devoted to station calibration and ice studies.  Both involved using a pulser, which generates a very short pulse, connected to a LPDA that transmits the pulse.  For the station calibrations, the stations' own antennas and electronics were used to digitize the transmitted pulse.  We managed to collect a large amount of station calibration data for a multitude of different transmitting antenna locations and tine orientations relative to the configuration of the stations' receiving antennas.  Since the received signal was fully propagated through the full electronics chain, as a neutrino signal would, these calibration studies will ultimately help quantify how well we can reconstruct the point in the sky the neutrino arrived from.
 


ARIANNA is in Antarctica because it needs vast amounts of high quality ice to act as a neutrino target and signal propagation medium.  When using the ice nature has pre-made, we are in a sense stuck with what we've got.  We can't change the ice properties, so have to fully understand them in order to accurately predict the shape of the neutrino signal we should expect.  That is the main purpose of the ice studies.  The idea is simple.  We transmit a well understood and reproducible pulse (with pulser and a LPDA) and study what we receive (with an oscilloscope and another LPDA).  The received signal has propagated down through the ice-sheet, reflected off the ice-water interface and then gone back up to the surface.  Thus, if we deconvolve the effect of the electronics and antenna from the received signal, any differences with the transmitted signal are due to properties of the ice.  To study the uniformity of the ice properties in the vicinity of the stations we repeated the same procedure of transmitting and receiving the signal but changed the location.  Then the received signals at the different locations can be compared and contrasted to look for ice difference.  Since we made the setup portable, we were able to move it at a number of different random locations away from camp.  These studies will confirm previous ice measurements establish the degree of uniformity of the properties of the ice at the ARIANNA site.



With our hard drives full of interesting data and the stations running great, it was time to break down camp and get back to McMurdo to take a hot shower that we had all felt desperate for.  Two days before our scheduled helo pick-up, a wolfer flew in to help us take the camp down.  With her help we made a berm of wooden crates full of equipment that is wintering over.  Everything was cleaned, packed and ready for the day of our departure, except our personal sleep tents.  On the day of the flight we took down the personal tents in time for a 3pm departure.  Unfortunately, the helo that was supposed to pick us up had mechanical issues and was grounded for the day.  That caused the helo schedule to fall behind.  We ended up being picked up around 9pm.  Luckily, that day was almost windless and sunny so spending the time outside wasn't a problem.  It was a good opportunity to reflect on our season, enjoy the scenic Transantarctic Mountains, and take lots of pictures.


 Many showers later, all of our stations are running great, and sending high-quality data we are in the process of analyzing.  The race to get ready for the next season has started and preparations for the 2015-2016 season are well on their way.  While preparing for another deployment, we are also hard at work analyzing data and summarizing physics results from the data we collected in a number of papers the ARIANNA collaboration will publish in the not so distant future, so stay tuned!
 



The 2014/2015 ARIANNA field season

The ARIANNA 7-station hexagonal array is now complete, thanks to an outstanding field season!  I didn't get to go on this, but here is a guest post from Joulien Tatar, who did.  The photos are by Chris Persichilli:





This was the last season for ARIANNA R&D work in Antarctica and it was a great one.  We successfully accomplished all of the tasks we had hoped to do this year and more.  ARIANNA is now ready to transition from the R&D phase by scaling up to the full array of ~1500 stations in an effort to detect cosmogenic neutrinos.

The deployment team this season (2014-2015) consisted of 5 physicists from UC Irvine:  Steve Barwick (PI), Corey Reed (Project Scientist: i.e. glorified postdoc :), James Walker (grad student), Chris Persichilli (grad student) and myself Joulien Tatar (postdoc).  Needless to say all of us were really excited to be going to Antarctica.  Chris and James were even more so since this was their first trip down. 


Even though preparation for the following season always begins as soon as we get back from the previous deployment, getting to Antarctica is much simpler than one would think.  NSF has excellent subcontractors who take care of the whole process (plane tickets, luggage, hotels, clothing, etc) of getting us safely to McMurdo, the main USAP base, and then back home.

Before flying with a USAP cargo plane (actually a US Air Force C-17) to McMurdo from Christchurch, New Zealand, we spent a couple of days going through safety briefings and getting appropriate clothing for the weather in Antarctica.  In between our scheduled tasks, we had time to explore Christchurch.  Steve was heartbroken.  He was last in Christchurch before the earthquake.  Most of the historic structures, charming coffee shops, lively bars and restaurants he used to frequent are gone.  However, I saw Christchurch come a long way from the rubbles it was in when I first visited in Nov. 2011, eight months after the major tremor.  Then, Downtown, where most of the structural damage took place, was completely fenced off and guarded by military personnel.  There was destruction everywhere you looked.  Now most of downtown has been rebuilt.  New businesses are opening throughout the city.  People have come back and things seem to be back to normal.  There is rarely chatter about the earthquake any more.

After a couple of days in Christchurch, we left for McMurdo.  Sometimes, the head winds are too high or the weather in McMurdo is poor for landing so the plane has to turn back.  This time we were fortunate to make it to McMurdo on our first try.  We spent about a week in McMurdo.  We had more briefings and training to go through while waiting for all of our cargo, shipped by boat, to make its way to McMurdo so we can leave for our field camp.  The station this year was at maximum capacity.  Many projects that were supposed to take place during the 2013-2014 season were delayed a year due of the government shutdown.  As a result, this year the USAP had to catch up and provide support for more projects than typical.



It took 6 helicopter flights to fly all of the components for the new stations and the rest of our electronics equipment.  Two wilderness first responders (wolfers) went to the ARIANNA site (in Moore's Bay on the Ross Ice Shelf) a day ahead of us to set up camp.  This was great because it allowed us to make every day count by starting work as soon as we arrived at camp.



Not long after we flew to the field camp, the wolfers went back to McMurdo.  The five of us, physicists, were alone on a remote Antarctic ice-sheet fully prepared to fend for ourselves and each other.  This is the first season we have camped without a wolfer.  The important help we were accustomed to getting from them was now all on us.  We had to make sure we always had melted snow for water, cleaned the two common tents (kitchen tent and science tent), check-in with McMurdo daily, cook, etc etc...



Cooking was one of the most time consuming and difficult chores we had.  We would each rotate to cook and clean for a day.  So one person would cook for everyone once every five days, which was not too bad.  Cooking in a small tent with a very limited amount of spices and ingredients (all provided to us from McMurdo) requires a certain amount of ingenuity it turns out we all possess.  The food we made was delectable and we managed not to burn down the tent.  We wrote down our food recipes so they can be used by the deployment team for years to come. :)



Our primary science objective was to have seven stations up and running and collecting high quality data.  That effort began by assembling station components that we did not ship pre-assembled.  The most time-consuming part of the assembly process was the power tower.  It consisted of putting together two ~10' triangular metal segments, mounting a 100W solar panel, and attaching two communication antennas.  It is as simple as it sounds and it took less than an hour to do.  Everything else (battery, antennas, electronics box with DAQ) came pre-assembled.  Once we had all of the components laid out and thoroughly tested, we were ready to take them to their final installation location.  Each station was placed at a corner of a hexagon and has a spacing of ~1km from the center of the hexagon where the seventh station (and our base camp) was located.  The transportation of the station was almost effortless, since we were given a sled we could load everything into and pull it with a snowmobile.  At a station's site we would first install the power tower and then dig four vertically oriented triangular slots to place the antennas in.  Digging these ~6' deep and 2' wide holes with a shovel was the most labor intensive and time consuming (1 hour) process of the station installation.  Once the holes were dug, we placed the Log-periodic Dipole Antennas (LPDA) in them, connected the various cables (power, communication, and LPDA) to the electronics box we placed at the base of the power tower, and had data streaming all the way back to our UC Irvine data server!  It took us less than 4 hours to install a station.  If we started installation a bit after breakfast, the station would be up and running before it was time for lunch.

To be continued in part 2...

Sunday, January 11, 2015

ANITA flies again



The ANITA balloon-born neutrino detection experiment has just finished its third flight.  It flew for  22 days, 9 hours, during which time it circled Antarctic about 1 1/3 times. 

ANITA floats high in the atmosphere (usually more than 100,000 feet), while it's 32 horn antennas look for radio waves from neutrino interactions in the Antarctic ice.  Because of its height, it can scan an enormous volume of ice, out to the horizon, up to 600 km away.  However, because of the distance to the interactions, it has a pretty high energy threshold, above 10^19 eV (roughly 100 times higher than ARIANNA).  In its previous two flights, it did not see any neutrino interactions, but it did set some of the best current limits on  ultra-high energy cosmic neutrinos.   They also observed pulses which they attribute to coming from cosmic-ray air showers.

For the third (and what was planned to be the last) flight, the collaboration made a number of improvements to increase the experiments sensitivity, including the addition of a large, lower-frequency antenna, which be stowed for take-off and then released in-flight to hang below the balloon.   

This flight was shorter than the previous flights; the second (ANITA-II) flight lasted 31 days, while the first flight was 35 days.  NASA has a nice web-page showing the ANITA flight track.   So, although the detector may have been more sensitive, this flight is unlikely to dramatically improve the overall ANITA sensitivity.

Katie Mulrey has written a couple of nice blog posts about the ANITA pre-flight preparations.  They are here and here

I'm eagerly looking forward to hearing more about how the flight went, and how the data looks.